That's mostly because of them using Musk's other business as leverage. A good company created explicitly to operate like this has no such vulnerability. The UK can try to stop them by trying to block the IPs or whatever, and the company is in turn free to try to circumvent it. The only issue is they may be banned from App store, which is a self-inflicted problem caused by Apple.
They simply blocked the whole of X in the whole of Brazil. They made it a crime to circumvent the block, so of course you could still access it with a VPN, as long as you didn't log into any account tied to your presence in Brazil, making it useless for politicians and the like. They didn't need to "use another business as leverage."